Method for bleaching mechanical and chemithermomechanical pulp g

ABSTRACT

The present invention relates to a method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said location, characterized in, that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.

TECHNICAL FIELD

[0001] The present invention relates to a method for bleaching mechanical and chemithermomechanical pulp (CTMP). Within the group mechanical pulps there are two dominating types of pulps, namely groundwood pulp (SGW) and thermomechanical pulp (TMP). In the manufacture of groundwood pulp roundwood logs are usually cut into lengths about one meter, which during water addition are pressed against a rotating grindstone. In the manufacture of thermomechanical pulp roundwood logs are usually chopped into a great amount of small wood pieces, named chips, and the true free-laying of the fibres is carried out by means of one or more refiners. Large parts of the chemithermomechanical pulp manufacturing process are similar to the thermomechanical pulp manufacturing process. The main difference is that one in a prestage treats the lignocellulose material, normally wood chips, with a sodium sulphite solution at a certain temperature and during a certain period of time. Consequently, the pulp yield will usually be one or some percent lower than in the case of thermomechanical pulp.

[0002] Any lignocellulose material whatsoever can be used as starting material for the pulps mentioned. Examples of such materials are bamboo, straw, bagasse, kenaf and wood. Wood is the preferred starting material, and both hardwood and softwood can be beneficially used, either separately or in combination.

BACKGROUND ART

[0003] When bleaching mechanical pulp and CTMP lignin-preservative bleaching is used. Among bleaching agents that can be used for lignin-preservative bleaching there are as well reducing bleaching agents as oxidizing bleaching agents. Within the first mentioned group dithionite, usually sodium dithionite, also called hydrosulfite, is the most common from a commercial point of view. Within the last mentioned group hydrogen peroxide is the most common from a commercial point of view.

[0004] The absolutely most common bleaching agent in the described connection seems to be hydrogen peroxide. The big advantage with hydrogen peroxide is that this agent is a very effective, i.e. a strongly brightening, bleaching agent. However, as a rule the bleaching with hydrogen peroxide demands that a separate bleaching tower and also other bleaching equipment have to be used, which leads to that the fixed costs at the hydrogen peroxide bleaching will be and are high.

[0005] Dithionite, usually sodium dithionite, is often not so effective from a bleaching point of view as hydrogen peroxide, but has the advantage, that it besides the use of a bleaching tower also can be added directly to the pulp suspension, for example in a storage tower or earlier in the pulp manufacturing process, for example just after the free-laying of the fibres, without the use of bleaching towers and other bleaching equipment. The last mentioned means a decrease of the fixed costs. This with adding a reducing bleaching agent, for example sodium dithionite, directly to the advancing pulp suspension just after the free-laying of the fibres is known from the Swedish patent application 9900816-1.

[0006] Transition metals, particularly iron and manganese, are detrimental to the bleaching of mechanical pulp and CTMP with, e.g. as well hydrogen peroxide as dithionite. The presence of mangane ions in significant quantities is particularly serious when bleaching of such pulp with hydrogen peroxide. An increasing amount of manganese ions leads to an increasing decomposition of the added amount of hydrogen peroxide which means, that the part of the hydrogen peroxide which is decomposed comes not to a use for brightening the pulp. Furthermore the decomposed products are in themselves harmful, because they form chromophore groups in the pulp, which works against that which is tried to attain, namely a brightening of the pulp. When bleaching of such a pulp with dithionite there are the iron ions, which are particularly dangerous. These transition metals are normally removed from or neutralized in the pulp and the pulp suspension by complex-binding the transition metals with a complexing agent, for instance in the form of ethylene diamine tetraacetic acid (EDTA) and/or diethylene triamine pentaacetic acid (DTPA). It has also been suggested that a reducing chemical such as sodium hydrosulphite or sodium sulphite for instance, is added to the pulp suspension in addition to a complexing agent. Success has also been achieved by treating the starting material, usually wood chips, with a complexing agent solely or with both of the aforesaid chemicals.

[0007] Use of the reducing bleaching agent borohydride, usually sodium borohydride, for bleaching of for example high yield pulp has also been suggested. In the U.S. Pat. No. 3,100,732 it is learned out about use of alkali metal borohydride and a peroxide bleaching agent in the same stage. Two ways of carrying out such a bleaching of the pulp are described. According to one of the ways a common bleaching agent solution is prepared which is added to the pulp, i.e. sodium borohydride is dissolved in a solution of peroxide, which also can contain water glass (sodium silicate), magnesium sulfate and sodium hydroxide. According to the other way a sodium borohydride solution is added to the pulp at the end of the peroxide bleaching stage in order to use the residue peroxide in an effective way at the finish of the bleaching stage. According to both ways an increase in brightness was obtained in comparison with that solely peroxide was used as a bleaching agent. A drawback with borohydride is their high price.

DISCLOSURE OF THE INVENTION

[0008] Technical Problem

[0009] In the manufacture of bleached mechanical pulp like TMP, for example from fresh Scandinavian spruce wood, it is today normal to achieve a brightness of 80% ISO and possibly one or two percent units above that. To achieve pulp brightnesses that are higher than those just mentioned, for example up against 85% ISO, is not possible with the known technique.

[0010] The Solution

[0011] The present invention provides a solution to this problem and relates to a method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said adding location, characterized in, that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.

[0012] Among the earlier enumerated pulps TMP is a dominating pulp type from a quantitative point of view. In the manufacture of such a pulp the process for laying the fibres free is usually carried out either in one refiner or in two subsequent refiners. Subsequent to the refiner or subsequent to respectively refiner a steam separator is situated, ususally a cyclone of some kind, through which the obtained pulp suspension is brought to pass. The pulp suspension is thereafter transported usually to a slusher (which however is not absolutely necessary), for example by means of a screw conveyor, and further on to a storage vessel (latency chest) and therefrom to a screening department and from there to a bleaching treatment. In certain TMP-mills there is a slusher and the pulp suspension is transported through a conduit to the storage vessel by means of a pump, located just after the slusher.

[0013] There are a number of alternative locations for adding the reducing bleaching agent, usually in the form of a water solution, to the advancing pulp suspension. The bleaching agent can, for example, be added to the pulp suspension in the earlier mentioned screw conveyor. One other adding location and a preferred one is the earlier mentioned pump. One possibly obtains a superior distribution of the bleaching agent in the pulp suspension as a result of adding the bleaching agent in that pump. White water is normally delivered to the slusher and the bleaching agent can be introduced in that white water, which later on is introduced into the pulp suspension. It is also possible, to introduce the bleaching agent directly into the slusher. It is of course possible to divide the charge of bleaching agent and to add the bleaching agent to the pulp suspension, for example in two or more of the stated locations.

[0014] The phrase “the advancing pulp suspension” used in the foregoing and also in the main claim shall be given a wide meaning. This phrase shall not solely be seen to mean when the pulp suspension flows forwards in a conduit or pipe, but also when the pulp suspension is held in a vessel and container, for instance in the form of a slusher and storage vessel since even in these latter cases the pulp suspension still moves forwards in the sense that it is fed into the vessel at one location and exits from said vessel in another location.

[0015] Any known reducing bleaching agent can be used. Examples of such bleaching agents are dithionite (which is sometimes called hydrosulfite and which is preferred), borohydride, hydrazine and formamidine sulfuric acid. Dithionite is commercially available primarily as sodium dithionite, i.e. Na₂S₂O₄. The bleaching agent concerned is introduced into the pulp suspension primarily in the form of an aqueous solution, the concentration of which will suitably lie within the range of 20-120 g/l. The amount of bleaching agent added will depend, among other things, on the difficulty in bleaching the pulp in question and how big increase in brightness of the pulp which is desired in the prebleaching in comparison with the increase in brightness of the pulp totally.

[0016] Pulp bleaching parametres such as temperature, time, pulp consistency, pH, etc., are mainly determined by the conditions that prevail naturally when producing thermomechanical pulp (TWP), as in the described case. At the aforesaid locations at which the bleaching agent is added to the pulp suspension, the temperature will, of course, be very high, e.g. 80-95° C., and the consistency of the pulp normally low, e.g. 2-4%. The bleaching time will be short as a result of this very high temperature among other things, and will probably range from a time span of some seconds up to some minutes. The bleaching time will probably also depend partly on the rate at which the pulp suspension flows at the location where the bleaching agent is added. The pH-value will naturally lie within the range of 4-7. In some cases, it may be advisable to adjust the pH-value by adding either an acid or an alkali to the pulp suspension at the location concerned. When using dithionite as a bleaching agent, the pH-value should lie from 4.5 and upwards, in order to achieve an optimal bleaching result. Although a pH-value as high as 8.5 can be used for bleaching purposes, a pH of this magnitude is less suitable for other reasons.

[0017] The amount of oxygen in the pulp suspension shall at the addition of the bleaching agent be as low as possible, preferably zero. The pulp suspension is protected from air contact and air dispersion in a considerable degree from the location of laying the fibres free and forward some length in the chain of pulp treatment. Besides that the temperature of the pulp suspension is very high in that area, which also contributes to an effectively bleaching of the pulp.

[0018] Manufacture of CTMP is very similar to manufacture of thermomechanical pulp, which has been described briefly above and the locations for addition of the bleaching agent at prebleaching of such a pulp coincident to a high degree with the above described.

[0019] When manufacturing groundwood pulp the pulp suspension is collected in a grinder pit after that the fibres are laid free, wherefrom the pulp suspension is transported forward. Suitable locations for addition of the bleaching agent are the grinding pit and just subsequent to that. It is especially suitable to carry out the bleaching in question in a system pressurized with steam, i.e. at pressure grinding.

[0020] This prebleaching of pulp is described in detail in our Swedish patent application No. 9900816-1. In this patent application it is also described how the reject pulp flow is taken care of and preferably is bleached before the reject pulp flow is mixed into the main pulp flow.

[0021] Subsequent to screening (usually) and possibly passing a buffer vessel and/or a storage tower the pulp is subjected to at least one further bleaching treatment.

[0022] Either directly or after the bleaching stage laying therebetween, the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.

[0023] In the determination reinforced with borohydride is included as well that the pulp is treated with borohydride in a pretreatment stage, i.e. before the true peroxide bleaching stage, as that borohydride is added to the pulp some time during the true peroxide bleaching stage, including that the borohydride is added to the pulp suspension together with the peroxide bleaching agent and that the borohydride is added to the pulp suspension at the end of the peroxide bleaching stage.

[0024] The peroxide bleaching stage can in itself be conventional (except a possible addition of borohydride) and any known peroxide bleaching agent can be used. Examples of such are hydrogen peroxide, sodium peroxide, peracetic acid, performic acid and peroxosulphuric acid (Caro's acid). Hydrogen peroxide is the preferred peroxide bleaching agent. The bleaching time is at least 30 minutes, preferably at least 60 minutes and can, for example, reach 2 hours, i.e. 120 minutes. The parametres otherwise are; charge of peroxide, counted as hydrogen peroxide,=0.5-5%, counted on bone-dry pulp, temperature=50-90° C., pulp consistency=5-38%, start-pH in the case of hydrogen peroxide=at least 10.

[0025] Among the ways of reinforcement with borohydride described above the use of a pretreatment of the pulp is the absolutely preferred.

[0026] Any known alkali metal borohydride can be added to the pulp. The most common is sodium borohydride. This can be added as such to the pulp. In practice the commodity Borol, which is a water solution containing 12% sodium borohydride and 40% sodium hydroxide, is the most used. It is an advantage to use one or more completion chemical(s) to the borohydride during the pretreatment stage. In the case the commodity Borol is used as a borohydride source it is not always necessary to add an alkali, e.g. sodium hydroxide, to the pulp, in view of that the commodity contains a considerable amount of sodium hydroxide. However, it is usual to add variable amounts of alkali to the pulp, for example together with the commodity Borol. In the case that sodium borohydride as such is added to the pulp it is necessary with an addition of alkali also; e.g. sodium hydroxide. Further suitable completion chemicals are water glass (sodium silicate) and some magnesium compound, as magnesium sulfate. The pretreatment of the pulp with at least borohydride takes place during a short period of time, for example at least 15 seconds and at most 500 seconds. Optimal result is obtained within the interval 60 to 180 seconds. The time period in question is counted from when the borohydride is added to the pulp up to that the peroxide bleaching agent is added to the pulp in the subsequent peroxide bleaching stage. It is suitable that the borohydride, counted as sodium borohydride, is added to the pulp in an amount of 0.05-0.3%, counted on bone-dry pulp. Suitable temperature is 20-95° C. The pulp consistency is not directly critical, but it is preferred with from medium consistency and higher, i.e. from 6% to 38%. While alkali always is present during the pretreatment of the pulp via the addition of the commodity Borol or via a direct addition or via both the ways the pretreatment will be carried out under alkaline conditions, for example within the pH-interval 9-13.

[0027] At the subsequent peroxide bleaching of the pulp it is in some cases suitable with addition of completion chemical(s) and in some case also necessary. If it in the preferred case with hydrogen peroxide as bleaching agent is not added any extra alkali or too little alkali in the pretreating stage it is necessary with an addition of alkali, e.g. sodium hydroxide, in the peroxide bleaching stage. It can be good with an addition of water glass and some magnesium compound also, especially if these chemicals are not added at all or are added in a too low amount to the pulp in the pretreatment stage.

[0028] The treatment of the pulp described above, i.e. this especially technique of bleaching is in detail described in a parallel patent application, with the same filing date as the present patent application. That Swedish patent application has the number 0000425-9.

[0029] It is possible (as earlier stated) to mix the borohydride and the peroxide bleaching agent and any completion chemicals in one and the same water solution forming one bleaching liquor which is added to the pulp suspension leading to a bleaching treatment of the pulp suspension in correspondence with earlier described peroxide bleaching stage.

[0030] Furthermore it is possible to add to the pulp suspension borohydride and any completion chemical at the end of, for example a conventional peroxide bleaching stage, for example, when the peroxide bleaching has been run in 100-120 minutes.

[0031] The charge of borohydride, counted as sodium borohydride, can also in these two cases lie within the interval 0.05-0.3%, counted on bone-dry pulp.

[0032] As earlier stated the pulp suspension just before the bleaching with peroxide bleaching agent reinforced with borohydride can be bleached in some other way in one or more stage(s). According to a preferred embodiment of the invention the in the earlier described way prebleached pulp is bleached with peroxide bleaching agent at medium consistency, while the subsequent peroxide bleaching stage reinforced with borohydride is carried out at high pulp consistency and furthermore the spent bleaching liquor from the high consistency bleaching stage, which contains residue peroxide like residue hydrogen peroxide, shall be delivered backwards in the pulp refining process and be mixed into the pulp just before or in the beginning of the medium consistency bleaching stage with e.g. hydrogen peroxide.

[0033] Advantages

[0034] By bleaching mechanical pulp and CTMP in the described way, i.e. in accordance with the invention, it is possible to achieve a brightness of the pulp of up against 85% ISO. The use of usually three different bleaching agents leads, however, to an increase in the price of the bleaching process seen in total, in comparison with a conventional use of one bleaching agent in usually one stage, but one has then to remember, that the market is always asking for an increased brightness of the pulps of the described kinds and consequently also for increased brightness of paper manufactured from such pulps. This demand has as a natural result, that the market is prepared to pay substantially for that which is demanded.

[0035] It is also so, that even if three different bleaching agents are used in the method according to the invention leading to a substantial cost for the bleaching agents the bleaching process seen in total is most cost effectively, among other things of the reason that the inventive different stages are associated with low fixed costs.

[0036] Best Embodiment

[0037] Since the invention already has been described in detail and even in a very detailed way with reference to two other patent documents that limitation is done here, that only one working example will be presented.

EXAMPLE 1

[0038] Thermomechanical pulp was manufactured in a way, which schematically is presented in FIG. 1 in the Swedish patent application 9900816-1, to which is referred once more. The starting material for the pulp manufacturing was considerable fresh, barked spruce logs of Scandinavian origin. To the advancing pulp suspension in location 14 and more precisely in the pump (not shown in the Figure) just after the slusher 14 a water solution containing sodium dithionite in a concentration of 60 g/l is added in such a flow, that the charge of the bleaching agent was 6 kg per tonne of bone-dry pulp. The temperature of the pulp suspension in that location was 88° C., their pH-value 4.6 and their pulp consistency 3%. The pulp prebleached in that way was screened in the screening department 19 so that an accept pulp flow and a reject pulp flow were obtained. The reject pulp flow was treated in accordance with that presented in FIG. 1 in the patent application mentioned and a water solution containing sodium dithionite in a concentration of 60 g/l was added to the reject pulp suspension flow in the pump (not shown in the Figure) situated just after the slusher 31 in such a flow, that the charge of the bleaching agent was 6 kg per tonne dry pulp also in that location. The temperature of the pulp suspension in that location was 85° C., their pH-value 5.1 and their pulp consistency 3%.

[0039] On the disc filter 21 pulp was picked out for further transportation to the laboratory. In that location the pulp, i.e. the prebleached pulp, had a brightness of 71% ISO and a metal content, determined according to standard analysis methods, of 12.1 ppm (or mg per kg bone-dry pulp) manganese (Mn), 11.2 ppm iron (Fe) and 3.8 ppm copper (Cu). The pulp arrived to the laboratory was pressed to a pulp consistency of 35% and was thereafter stored in a cold-storage room.

[0040] Four trials were done with this pulp in the laboratory, one reference trial and three trials where the method according to the invention was simulated.

[0041] At the runs of trial pulp was taken from the cold-storage room and was divided into charges of 20 gram bone-dry pulp to every trial.

[0042] At the reference trial the pulp was subjected to a conventional hydrogen peroxid bleaching in the way that the 20 grams of pulp was divided finely by hand so that the pulp become fluffed. Thereafter the pulp was put down into a plastic bag. To this plastic bag a certain amount of a water solution containing hydrogen peroxide in a charge of 4%, counted on bone-dry pulp, sodium hydroxide in a charge of 3.5%, counted on bone-dry pulp, and water glass (sodium silicate) in a charge of 3.12%, counted on bone-dry pulp was delivered. After the addition of the mentioned water solution the content of the plastic bag, i.e. the pulp sample, was kneaded by hand so that the chemicals were distributed equal in the pulp amount stated. The pulp consistency was 30% after mixing the chemicals into the pulp. Thereafter the plastic bag with its content was placed into a water bath, with a temperature of 75° C., during a time period of 90 minutes.

[0043] The bleaching was interrupted by picking the plastic bag up from the water bath and its content was moved to a glass vessel, in which the pulp sample was diluted with deionized water down to a pulp consistency of 0.6%. A sulphuric acid solution was also added so that the pH-value of the pulp suspension was decreased to 5. During the dilution of the pulp sample and before the addition of sulphuric acid a specimen of the spent bleaching liquor was taken out in which the end-pH and the content of hydrogen peroxide residue were determined according to conventional analysis technic. It was manufactured a sheet by hand of the pulp sample according to SCAN-CM 11:75 on which 30 the brightness was determined according to the analysis method SCAN-P 3:93. All the brightness values mentioned in this working example are measured in the just described way.

[0044] Also the three bleaching trials according to the invention were carried out on 20 grams bone-dry pulp.

[0045] In one of these trials, named Pr, at first a water solution containing sodium borohydride and sodium hydroxide in the form of the commodity Borol in an amount regarding sodium borohydride of 0.1%, counted on bone-dry pulp, plus an extra amount of sodium hydroxide of 1.55%, counted on bone-dry pulp, and water glass in an amount of 3.12%, counted on bone-dry pulp, was added to the pulp sample in the plastic bag. After thoroughly dividing of the chemicals by means of hand kneading the sealed plastic bag was placed in the earlier mentioned water bath, which held a temperature of 75° C. After 2 minutes the plastic bag was picked up from the water bath and opened and a water solution containing hydrogen peroxide in an amount of 4%, counted on bone dry pulp, and sodium hydroxide in an amount of 1.55%, counted on bone-dry pulp, was delivered. After this addition of the water solution the pulp had a pulp consistency of 30%. The plastic bag was sealed and again placed in the water bath mentioned and was kept there for a time period of 90 minutes. Also in this trial the handling of the pulp sample was continued in the way described above. The characterizing thing for this trial was obviously that the sodium borohydride was added to the pulp in a pretreatment stage before that the true hydrogen peroxide bleaching of the pulp was carried out.

[0046] In a second trial according to the invention, named PS the pulp sample was treated in accordance with the reference trial with the important difference, that the pulp beside the in that trial mentioned chemicals also was fed with the commodity Borol, so that the charge of sodium borohydride was 0.1%, counted on bone-dry pulp. Furthermore the direct charge of sodium hydroxide was decreased to 3.1%, counted on bone-dry pulp. The characterizing thing for this trial was obviously, that the sodium borohydride and the hydrogen peroxide were added to the pulp at the same time.

[0047] In a third trial according to the invention, named P_(s), the pulp sample was treated in accordance with the reference trial with the important difference, that after 80 minutes of the hydrogen peroxide bleaching of the pulp the plastic bag was picked up from the water bath and opened. To the bag the commodity Borol was delivered so, that sodium borohydride was added to the pulp in an amount of 0.1%, counted on bone-dry pulp and furthermore deionized water was added so that the pulp consistency was decreased to 5%.

[0048] Sulphuric acid was also added so that the pH-value in the pulp suspension was decreased to the interval 6.5-7.0. The plastic bag was sealed and the chemicals were equilized within the pulp sample by means of hand kneading. Thereafter the plastic bag was again placed in the water bath during a time period of 10 minutes. Also in this trial the direct charge of sodium hydroxide was decreased to 3.1%, counted on bone-dry pulp. The characterizing thing for this trial was obviously, that the sodium borohydride was added to the pulp just at the end of the hydrogen peroxide bleaching.

[0049] Trial parameters and results achieved are clear from the Table 1 below. TABLE 1 Trial Parametres and results Reference P_(r) P_(s) P_(e) NaBH₄, % 0 0.1 0.1 0.1 Na₂SiO₃, % 3.12 3.12 3.12 3.12 NaOH, % 3.5 3.1 3.1 3.1 H₂O₂, % 4.0 4.0 4.0 4.0 Pulp consistency, % 30 30 30 30;5 Temperature, ° C. 75 75 75 75 Time, minutes 90 90 90 80 + 10 End-pH 9.16 9.17 9.37 — H₂O₂ residue, % 0.56 1.00 0.60 1.12 Brightness, % ISO 81.6 83.6 82.4 82.2

[0050] As illustrated all the pulp samples which have been bleached according to the invention are given brightnesses, which are higher than the brightness of the reference pulp. In the trial where an explicit pretreatment stage of the pulp with borohydride has been used the increase in brightness is especially pronounced and more exactly 2% ISO.

[0051] Furthermore it has afterwards been verified that at the occasion when prebleached pulp was picked out in the TMP-mill and was delivered to the laboratory it happened to be so that the brightness of the initial pulp, i.e just after the second refiner stage, was unusually low, which leads to a lower end brightness of the pulp than what under other circumstances should be the case. 

1. Method for bleaching mechanical and chemithermomechanical pulp including that an advancing pulp suspension obtained after that the fibres are laid free is prebleached by adding reductive bleaching agent to the pulp suspension in a location just after the fibres are laid free and that the bleaching is carried out under given conditions in the form of high temperature and minimized oxygen access in respect of said adding location and immediately downstream of said location, characterized in that the in the described manner prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.
 2. Method according to claim 1, characterized in, that borohydride is added to the pulp and is permitted to react with this during a short period of time in the form of a pretreatment before the pulp is bleached with peroxide bleaching agent.
 3. Method according to claim 1, characterized in, that borohydride is added to the pulp at the beginning of the peroxide bleaching treatment.
 4. Method according to claim 1 characterized in that borohydride is added to the pulp at the end of the peroxide bleaching treatment.
 5. Method according to claims 1-4, characterized in, that the prebleached pulp is bleached with peroxide bleaching agent and thereafter with peroxide bleaching agent reinforced with borohydride.
 6. Method according to claim 5, characterized in, that the first peroxide bleaching stage is carried out at medium consistency and that the second peroxide bleaching stage is carried out at high pulp consistency and that spent liquor containing residue peroxide from the second bleaching stage is returned and mixed into the pulp before or in connection with the first peroxide bleaching stage.
 7. Method according to claims 1-6, characterized in, that to the pulp is delivered at least one of the chemicals alkali, water glass (sodium silicate) and magnesium compound as completion to borohydride.
 8. Method according to claims 1-7, characterized in, that to the pulp is delivered at least one of the chemicals alkali, water glass (sodium silicate) and magnesium compound as completion to peroxide bleaching agent.
 9. Method according to claim 1, characterized in, that the reductive bleaching agent is sodium dithionite.
 10. Method according to claims 1-9, characterized in, that the borohydride is sodium borohydride, preferably in the form of the commodity Borol.
 11. Method according to claims 1-10 characterized in, that the peroxide bleaching agent is hydrogen peroxide. 